The present invention relates to a printing head of a dot line printer, and more particularly, to an improvement on tip ends construction of printing pins of print hammers in the printing head in which a plurality of printing pins are provided in a shuttling direction as well as in a line to line direction. Throughout the specification, the term "shuttling direction" indicates a transverse direction of a printing sheet or reciprocating or shuttling direction of the printing head so as to provide a dot printing line(s) in order to provide a character line, and the term "line to line direction" indicates feeding direction of the printing sheet.
In dot line printer, a printing head has a plurality of print hammers juxtaposedly arranged at a predetermined intervals in the shuttling direction. Upon reciprocating motion or shuttling motion of the printing head, the printing hammers are driven to obtain dot printings. More specifically, as shown in FIGS. 1 and 2, the printing head includes a plurality of printing hammers 10 provided side by side in the shuttling direction. Each of the printing hammers 10 includes a hammer springs 12, a printing pin 11 provided at free end of the hammer spring 12 and a plunger 13 provided on the hammer spring 12 but positioned more adjacent to a base end of the hammer spring 12 than the printing pin 11. The plungers 13 are selectively attracted by corresponding yokes (not shown), so that printing pins 11 are selectively moved toward the printing sheet to perform dot printing through an ink ribbon (not shown).
Shapes of the printing pins 11 and position alignment of the printing pins 11 are extremely important in order to provide a high quality print image. To this effect, the printing pins 11 undergo provisional machining or grinding so as to obtain a generally circular cross-section prior to their assembly to the corresponding hammer springs 12, and in the printing hammers shown in FIGS. 1 and 2, the printing pins 11 are arrayed in the shuttling direction, so that a single dot line is provided during one shuttling motion of the printing head. In this connection, accuracy of the attachment positions of the printing pins 11 relative to the hammer springs 12 is also very important.
For the assembly of the printing pin 11 to the hammer spring 12, as shown in FIG. 3, the provisionally machined printing pin 11 is inserted into a hole 12a of the hammer spring 12, and then the printing pin 11 is subjected to caulking to fixedly secure the pin 11 to the spring 12. Thus, accuracy of pin alignment is determinative by the positioning and caulking works of the printing pins 11. Alternatively, as shown in FIG. 4, the printing pin 11 provisionally subjected to machining or grinding is integrally connected to the hammer spring 12 by brazing. Thus, accuracy of the pin alignment is determinative by the brazing position or work.
According to the above described method for producing the printing hammers, the intended pin alignment may not be easily obtained due to several reasons. For example, if the position or the hole 12a or machining accuracy of the hole 12a is not appropriate, the pin alignment may be degraded. This degradation may also occur due to inaccuracy in caulking. For example, the pins 11 may not have the proper orientation with respect to the hammer spring 12. Further pin alignment is also degraded, due to inaccuracy in brazing. For example, it would be rather difficult to obtain a correct brazing position.
In order to improve the pin alignment, another method has been proposed through inhouse R & D activities in such a manner that the machining or grinding to the printing pins 11 is carried out after these are assembled, with the hammer springs 12. For example, as shown in FIGS. 5 and 6, the free end portions of the hammer springs are bent, and then, the bent end portions are subjected to machining or grinding so as to obtain rectangular printing pin face 11a in which two sides are directed in parallel with the shuttling direction indicated by an arrow X, and the remaining two sides are directed in the line to line direction indicated by an arrow Y as shown in FIGS. 7 and 8. By the subsequent grinding, operation the resultant printing pin has a truncated pyramid shape.
The grinding is performable since the grinding of the one printing pin does not affect the shaping of the other or neighboring printing pins because of the linear array of the printing pins in the shuttling direction. As a result, each two sides of the rectangular pin face 11a of the printing pins are directed in parallel with the shuttling direction (Of course, the remaining two sides of the rectangular face 11a are directed in the sheet feeding direction). However, this grinding is unavailable for a second type of printing hammers in which the printing pins on the printing hammers are positioned offset from one another in the sheet feeding direction as shown in FIG. 9. More specifically, in the second type of printing hammers, in an attempt to perform high speed dot line printing, the above offsetting arrangement can simultaneously provide multiple dot lines in one shuttling motion of the printing head (of course, several shuttling motions can provide a single character line by the combination of the multiple dot lines). Further, dot pitch is also reduced in the shuttling direction, so that high density in the printing pins results in this direction.